Thermionic valve amplifier



Feb. 25, 1947. M. M. LEVY THERMIONIC VALVE AMPLIFIER Filed March 24,1942 Lam r02 Arro/e/vcfr Patented Feb. 25, 1947 THERNIONIC VALVEAMPLIFIER,

Maurice Moise Lvy, London W. C. 2, England, assignor to InternationalStandard Electric Corporation, New York, N. Y.

Application March 24,1942, Serial No. 435,983 In Great Britain April 7,1941 9 Claims.

This invention relates to improvements in amplifier circuits usingpentode Valves. vit concerns the reduction of the effect of certainstray capacities associated with such valves.

One of the causes which limits the efiiciencyoi an amplifying valve athigh frequencies is the capacity associated with the plate and itsconnections, which shunts the load connected in the plate circuit. Inpentodes-a large proportion of as cathode-followers. It is alsoapplicable to circuits in which the valves operate on the nonlinearparts of the characteristics, for-example, to circuits for amplifyingimpulses.

The principal object of the invention therefore is to improve theoperation of pentode amplifiers at high frequencies by the elimination,partly or wholly, of the effect of the'stray capacities associated withthe platecircuit of the pentode valves. This obiect is accomplished byconnecting the suppressor grid of any such pentode to some point in ,theamplifier, the alternating potential of which is equal to or nearlycanal to the alternating potential of the plateof the pentode. In thisway the terminals of the capacity between the plate and the suppressorgrid'ivvh ch frequently is a large fraction of the total plate capacity)are maintained at nearly the same potential. Thus the current whichwould flow through this condenser is reducedto a verysmall value.

According to another aspect, the suppressor grid is made to screen theplate from earth, and the princi le may be extended by supplying theexternal connections to the plate of the ,pentode with a screen which isconnected to the suppressor grid, and in this way thecapacities'associated with the external circuit are also eliminated orreduced. The efiect of this process may be regarded as equivalent toconnecting across the unwanted capacity a negative capacity by which itis neutralised.

According to certain special features of the invention, the point in theamplifier to which thesuppressor grid should be connected shouldbetweenthe point and earth :is relatively low in order that the-capacitywhich will the introduced by the connection of the suppressor grid andthe external screen, if any,may have aninappreciable effect.

The phase of the alternating potential of the point to which the.suppressor grid is connected shouldpreferably be .the same as the phaseof the alternating potential whichappears on the plate of the .pentodein .order that the corresponding difference-ofpotential'shall atall-times be small. It mayhappen that .the'point to which it is vdesiredto connect .thesuppressor grid has the op posite phase in which caseconnectionis made through a transformer, the windings of which areconnected in such a direction that the desired phase isproduced on thesuppressor grid. 7

According .to .another feature, .the alternating potential of thesuppressor grid may also be chosen to be higher than that .of the .plateof the pentodein which case additional current is fed through the straycapacity concerned. This enables other straycapacities associated withthe plate impedance to be neutralised. vBy another variation, the phaseof the alternating potential applied to the suppressor grid may bechosen to have :any relation .withrespect to the phase of the plate andby this the-effect of shunting the capacity between theplateandsuppressor grid by any impedance, positive .or negative, can beproduced. According to an rembodiment of the invention, an amplifierconsists of-afirst pentode-stage followed by other stages comprisingvalves connected as cathode followers. The suppressor grid of thepentode is connected to the cathode of the following valve, the ACpotential of which will generally-be very nearly the same as the'potential of the plate of the pentode. Furthermore,

the impedance to ground of this cathode is usually small compared "withthe 'impedance to ground of the plate-of the pentode. In this-way theprinciples of the invention which have just been explained are carriedout. The same arrangement may be adopted for *any pentode in any of thesubsequent stages of the amplifier.

In another embodiment, the amplifier come prises some stages withnegative cathode feedback. The cathode of any valve is then connected:to the suppressor grid of the preceding valve. "In this way, thecapacity between the plate andrsuppressorgrid oicertain of the valvesmay be reduced by *an amount depending upon the value of the negativefeedback of the corresponding ,following valve.

These-features of the invention will be .more

the suppressor grid of a pentode valve inaccord- V ance with theinvention; v I

Fig. 4 shows an embodiment in which a pentode valve is followed by avalve connected as a oathode follower;

Fig. 4A shows a modification of the embodi-- ment shown in Fig, 4.

Fig. shows the circuit of an amplifier hav-.

ing stages with negative reaction.

In Fig. 1 is shown a pentode valve V! which may form part of anamplifying circuit. The load impedance Z of the plate circuit is shuntedby a series of stray capacities which limit the amplification of thecircuit at high frequencies. These capacities can be convenientlydivided into two portions; CI which represents the capacity between theplate and the suppressor grid, and

C2 which represents the capacity to .ground of the plate (exclusive ofCI and of the connecting leads. Fig. 1 is intended to show the circuiteffective as regards the amplification of high frequency voltages, whichmay be assumed to be. applied to the control grid. The arrangements forpolarising the plate and grids and other details are accordinglyomitted. v

The effect .ofthe capacity CI can be reduced if a varying potential, inphase with the plate potential producedgby thesignalapplied to the grid,be connected to the suppressor grid. If" this varying potential benearly equal to the plate potential, then the voltage across thecondenser CI will always be small and its effect will thus be greatlyreduced.

This will be made clearer by reference toFig. 2

which shows the essential circuit characteristics of Fig. 1. The pointsA and S correspond in the two circuits, The plate circuitis representedby a generator GPin series with, an impedance ZP corresponding to theinternal output impedance of the valve.

will be small or zero. The corresponding voltage drop produced in Z? bythe current in CI will thus be practically eliminated. Further. if thepoint is can be brought to a potential higher than 1 that of A, currentwill flow in the reverse direction in CI from GS. This means that extracur- I rent will be supplied to Z by which the eifect of C2 (and perhapsalso of other similar stray capacities introduced in Z) will beneutralised.

The effect of the generator can be regarded as equivalent to connectinga negative capacity parallel with Cl, whose value depends upon thevoltage of the generator. It is also possible to produce the effect ofconnecting various kinds of 1 I negative or positive impedances inparallel with CI by'suitably choosing the amplitude and phase I of theoutput of the generator GS with respect to GP.

ThegeneratorGS in series with ZS represents the effect of connectingthevary- 4 a point of constant potential (normally equal to ornear thatof the cathode) in order to maintain it at a potential sufiiciently lowwith respect to the plate to suppress the secondary electrons producedthereat. The potential difference between the plate and the suppressorgrid will thus usually be equal to or a little less than the appliedplate voltage. Thealternating plate potential in an i amplifying valvedue to the signal is however generally small compared with the appliedplate Voltage, and accordingly the suppressor grid can be connected to apoint of alternating potential without substantially affecting itsfunction in suppressing the secondary electrons, so long as thisalternating potential is of the same order as the alternating platepotential. In general this will be the case and in the most interestingcase when the potentials of the plate and suppressor grid Vary in phasefor eliminating the elfect of 0!, the difference of potential betweenthe plate and the suppressor grid will be maintained approximatelyconstant.

Fig. 3 shows schematically one way in which the desired varyingpotential may be obtained. The block A represents the remainder of theamplifier of which the valve V! is supposed to form a part. Theeffective ground capacity of the suppressor grid is represented by C3.

The required variable potential for the suppressor grid is preferablytaken from some'point 7 'PI in the amplifying circuit as shown where theimpedance to ground is sufficiently small for the eifect of the capacityC3 added in parallel to be unimportant. A particularly interesting caseis that in which the amplifier works into a low output impedance Z0. Ifthe output potential is in phase with the plate voltage of the valve VI7 the suppressor grid may be connected to the out- According to theusual method of. operating put at P2, for example. If there is phaseopposition, a transformer T may be connectedin front of Z0 as indicatedin Fig. 3A, and the suppressor grid can be connected to the output sideof the transformer at P2. The capacity G3 which now comes in parallelwith Z0 will have a negligible eifect because Z0 is small.

If it should be desired to connect the suppressor grid to an internalpoint such as P! (Fig. 3 where the phase is opposite to that necessaryto eliminate the effect of CI, a transformer may also be used in amanner similar to that just described. V

Fig. 4 shows the circuit of a two-stage amplifier which exhibits certainother aspects of the invention.' The stray capacities CI, C2 and C3 areshown in dotted lines associated with the first valve VI; The secondvalve V2 is shown connected as a cathode follower.

The suppressor grid of the valve V! is connected to the cathode of V2,and on account of the fact that V2 is arranged as a cathode follower,the potential of the suppressor grid of VI will be generally nearly thesame as the potential of the grid of V2. As this grid is coupled to theplate of VI through the condenser KI follows that the potential of thesuppressor grid of VI will always be very nearly the same as thepotential of the plate. Thus the effect of the capacity CI will bepractically eliminated.

Since V2 is connected as a cathode foilower, the cathode resistance RC2can be very small will accordingly be negligible because RCZissmall.

Some idea of the advantage gained by this arrangement will be obtainedfrom the following numerical example. If the valve Vi is a high slopepentode, the capacity between the plate earth will be about 9 a y. Thecapacity to earth of the grid of valve V2 might be about 12 12, butsince this valve is connected as a cathode follower the eifect of thiscapacity could be re duced to for instance, because the grid and thecathode will be nearly at the same potential. Accordingly the grid earthcapacity of V2 can be taken as effectivelyv equal to about 1.2 Thecapacity to earth of theexternal connections to the plate of VI cangenerally be arranged to be not more than about 2 u. Thus the totaleffective capacity to earth of the plate of VI would be that is, if thesuppressor grid of VI is not connected to the grid of V2, When, however,this connection is made, the effect of the capacity CI may be reduced toIn many valves, Ci will be 80% of the total plate-earth capacity thatis, about 7 say. Accordingly the capacity CI is effectively reduced toabout 0.7 m. The total capacity of the plate of VI now becomes that is,a little less than of the preceding value. With a given value, and withthe same circuit configuration, therefore, it would now be possible toincrease the plate resistance Z! twice, thereby increasing the gain by 6decibels.

It will be noted that the suppressor grid forms a partial screen betweenthe plate and earth. This screen may be extended as shown in Fig. LA toinclude the external connections to the plate by surrounding them by aconducting metal screen M which is connected to the suppressor grid.This screen could with advantage include also the condenser KI whichcouples the plate of VI to the grid of V2 in Fig. 4. This will have theeifect of suppressing part of the capacity C2 because most of it nowcomes in parallel with CI. The capacity of the screen which has beenadded will, of course, increase the capacity C3, but so long as thisincrease of capacity is not too great, or so long as the resistance RC2is sufficiently small, this will not introduce any objection.

In Fig. 5 is shown a three-stage amplifier containing valves connectedwith negative reaction and in which the capacity of CI is effectivelysuppressed in a manner very similar to that of Fig. 4. The first valveVI is connected as an ordinary amplifier without reaction and has ashunted biassing resistance GI connected in series with the cathode. Itis coupled to the next valve V2 by means of the plate resistance ZI andthe coupling condenser KI. Valve V2 is provided with a shunted biassingresistance G2 and also with a resistance RC2 in series to provide thenegative reaction. This valve is in turn coupled to the valve V3 bymeans of plate resistance Z2 and coupling condenser K2. The valve V3 issimilarly provided with a shunted biassing resistance G3 and resistanceRC3 to provide the negative reaction. The output is taken from valve V3across the plate resistance Z3 through coupling condenser K3. The straycapacities CI, C2 and C3 are shown connected to the valve VI, as before,by dotted lines. The suppressor grid of VI is connected to the cathodeof V2 as in Fig. 4 and the suppressor grid of V2 iscoupled to thecathodeof V3 by an alternative arrangement involving a shunt resistance RS anda coupling condenser KS, which has been shown by way of example. Thesuppression of the effect of capacity CI in the circuit of Fig. 5 occursin a manner similar to' effect of the capacity in valve V2 correspondingto CI will be reduced by valve V3 in exactly the same way, because asbefore the resistance RC3 will be much smaller than the plate resistanceZ2, and will not be affected by the alternative method of coupling thesuppressor grid which has been shown. The suppressor grid could alsohave been coupled to the cathode by other means such as. for example, bythe use of a transformer.

It will be seen that as regards the function of suppressing the effectof the capacity CI, the circuits of Figs. 4 and 5 are equivalent, butthe suppression in the case of Fig. 4 is likely to be more completebecause the grid potential is generally nearly the same as the cathodepotential while in Fig. 5 the grid potential will usually be somewhathigher than the cathode potential, de- 1 pending upon the amount ofreaction.

The arrangement which have been described are also applicable toamplifiers inwhich the valves operate on the non-linear portions of thecharacteristics. such as are used for amplitying impulses. The potentialof the cathode of any of the valves follows substantially thecorrespondin grid potential and so also the preceding plate potential assoon as the impulse reaches a certain voltage; the circuit then operatesas described.

The preceding circuits have been given only as examples, and variousother means for connecting the suppressor grid in accordance with theinvention can be imagined; for example, in an amplifier with feed-back.the feed-back circuit can be used for transmitting the desired potentiato the su pressor grid.

What is claimed is:

l. A thermionic amplifier system comprising amplification meansincluding a series of coupled amplifying stages having a tube in eachstage, the first stage tube being a pentode, and means for at leastpartially neutraliz ng capacity between the pentode anode and ground,comprising potential-transmitting coupling between the pentodesuppressor grid and a point in the amplifier system having analternating poten tial of the same frequency and phase as thealternating potential of said anode and a maxi mum value at least eoualto the maximum value of said anode potential.

2. A thermionic amplifier system comprising amplification meansincluding a series of coupled amplifying stages having a tube in eachstage, the first stage tube being a pentode, and means for. at leastpartially neutralizing capacity between the pentode anode and groundcomprising a potential-transmitting coupling between the pentodesuppressor grid and a point in a subsequent stage having an alternatingpotential of the same frequency as the alternating potential on saidanode, and of no lower peak value.

3. A thermionic amplifier system as set forth in claim 2, in which saidsuppressor grid is connected to the cathode of a tube in a subsequentstage, said cathode being at a potential difierent from groundpotential.

4. A thermionic amplifier system asset forth in claim 2, in which saidsuppressor grid is connected to the cathode of the tube in thesucceeding stage, and said succeeding stage has a negative reactioncoupling with the first stage.

5. A thermionic amplifier system as set forth in 'claim 2, in which saidsuppressor grid is connected to the output of a subsequent stage.

6. A thermionic amplifier system as set forth in claim 2, includingmeans for shifting the phase of the alternating potential applied tosaid suppressor grid.

7; A thermionic amplifier system as set forth in claim 2, includingmeans for reversing the phase of the alternating potential applied tosaid suppressor grid.

8. A thermionic amplifier system as set forth in claim 2, in which saidpoint in the amplifier system has a low impedance to earth,.thearrangementbeing such that the capacity introduced by said couplingbetween said point and said suppressor grid is'substantially withouteffeet on the operation of said subsequent stage.

9. A thermionic amplifier system as set forth in claim 2, in which saidcoupling comprises a shield connected to said suppressor grid, saidshield being in shielding relation to and capacitatively coupled withthe connection between the control grid of the subsequent stage and theanode of said pentode.

MAURICE MOISE LE'vY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

